Triggered by this question, a similar one. How can I write a will such that if a beneficiary is suffering from senile dementia at the time I die, they are passed elsewhere? The situation may well arise if I have previously succumbed likewise, and so cannot revise my will. My siblings are of a similar age, and share my genetic inheritance. I would want the money to go to their children, if they are beyond considering for themselves whether to enter into a deed to reassign their inheritance at the time of my death.
Is there any reason it can't be handled in the same way as survivorship? Specify a condition which, if satisfied, means that the beneficiary is to be treated as if they had pre-deceased me.